Integrated circuits have become more complex and smaller in scale as the applications for using integrated circuits have rapidly multiplied. Design requirements for such integrated circuits frequently exceed the capabilities of traditional silicon based semiconductor compounds. The search for higher performance, higher capacity and better frequency characteristics has resulted in developing new semiconductor compounds formed from elements in Group III and Group V of the Periodic Table. Combining elements from Group III and Group V has produced a new class of compound semiconductors which is particularly useful in high speed microwave and optical devices. Gallium (Group III) and arsenic (Group V) have been combined to produce gallium arsenide (GaAs) compound semiconductors.
A substantial amount of testing and development has been conducted with respect to gallium arsenide compound semiconductors. Various elements such as aluminum (Al) and indium (In) can be added to gallium arsenide to produce abrupt changes in band gap energies and refractive index. Aluminum gallium arsenide (AlGaAs) and gallium arsenide have nearly identical lattice constants which allows relatively thick layers (several microns) of aluminum gallium arsenide to grow on a gallium arsenide substrate. This characteristic results in gallium arsenide and aluminum gallium arsenide compounds being widely used in fabricating semiconductor devices. Indium gallium arsenide (InGaAs) has a significantly larger lattice constant as compared to gallium arsenide.
Another problem associated with present systems and methods for growing indium gallium arsenide epitaxy layers is that at high temperatures (500.degree. C. and greater), the indium gallium arsenide layer generally has a higher mole fraction of indium at the interface with gallium arsenide and a lower mole fraction of indium at the interface with aluminum gallium arsenide. This change in indium concentration, related to evaporation and desorption of indium at higher temperatures while growing epitaxy layers, produces undesirable electrical characteristics. Since indium gallium arsenide has a bigger lattice constant as compared to gallium arsenide, stresses are present at the interface bond between the indium gallium arsenide layer and the gallium arsenide substrate. Changes in the mole fraction of indium and discontinuities in the lattice structure at the bond interface have previously limited or minimized the use of indium gallium arsenide in preparing epitaxy layers for semiconductor devices.
The first step in fabrication of an integrated circuit on a semiconductor chip is to grow a relatively large, single crystal or ingot from the desired semiconductor compound. Various techniques are commercially available for growing a single crystal from gallium arsenide compounds. These techniques are sometimes referred to as bulk growth procedures.
A large, single crystal or ingot of gallium arsenide will typically have dimensions of three to five inches in diameter and two to three feet in length. This large crystal is then sliced into thin wafers which provide a crystalline substrate of compound semiconductor material. One or more thin film layers (sometimes referred to as "epitaxy layers") are then deposited on the crystalline substrate to produce the electrical characteristics associated with semiconductor devices and integrated circuits. Various compounds and elements may be included within the thin film layers to modify the band gap energy and the refractive index of the layers as compared with the semiconductor material in the substrate and with other layers on the substrate.
Various techniques have previously been used to place thin film layers on semiconductor substrates. Examples of thin film technologies which have previously been used include liquid phase epitaxy, chemical vapor deposition, sputtering and vacuum evaporation. Molecular Beam Epitaxy (MBE) has been found to be a particular useful technique for placing a thin film layer on compound semiconductors such as gallium arsenide. With an understanding of surface physics and by observing variations in surface structure resulting from the relationship between arrival of an atom (beam flux) and substrate temperature, MBE allows preparation of high quality, thin film layers by adding one atomic layer upon the next atomic layer. This type of thin film layer growth is particularly advantageous in fabricating gallium arsenide semiconductors.
General background information on molecular beam epitaxy with respect to development of semiconductor materials and particularly with respect to gallium arsenide may be found in "The Technology and Physics of Molecular Beam Epitaxy" by E. H. C. Parker, published by Plenum Press in 1985. This book teaches the use of a mass spectrometry detector to monitor background levels for selected atoms or molecules in an MBE System. Mass spectrometers used for this purpose are sometimes referred to as residual gas analyzer probes. Mass spectrometers have also been used in MBE Systems to detect undesired leaks from sources in the MBE System.
A substantial amount of work has been conducted to develop gallium arsenide semiconductors with epitaxy layers formed from aluminum gallium arsenide (AlGaAs) compounds. The use of indium gallium arsenide (InGaAs) as a ternary compound to grow an epitaxy layer on gallium arsenide substrates along with an aluminum gallium arsenide epitaxy layer offers substantial electrical advantages in integrated circuit design for fabrication on a semiconductor chip as compared to only an aluminum gallium arsenide epitaxy layer.
Therefore, a need has arisen for equipment and methods to grow high quality indium gallium arsenide epitaxy layers on a gallium arsenide substrate with uniform strain characteristics in the lattice structure and uniform mole fraction composition in the surface and subsurface layers of the semiconductor chip.